In general, layer s composed of organic semiconductors (also referred to as organic layer s) can be used for a variety of applications, e.g. for converting electrical energy into light or for converting light into electrical energy. By way of example, it is possible to produce organic light emitting diodes (OLEDs) which are used in electronic devices, e.g. in displays of TV sets, advertising panels or mobile radio devices, or as a planar light source for generating light.
Organic light emitting diodes (OLEDs) having high efficiency include a layer stack of many materials and material mixtures which fulfill different functions (e.g. transporting or injecting charge carriers, blocking individual charge carriers, trapping charge carrier pairs and forming excitons and subsequently emitting light). These multilayered OLEDs can be produced either by means of vacuum processing or by means of so-called hybrid processing, in which a plurality of layer s (e.g. up to three) are processed from a solution (i.e. as liquid phase) and the remaining layer s are processed in the vacuum. A limiting factor here for a multilayered solution-processed OLED (also referred to as liquid-phase-processed OLED) is that already existing layer s are incipiently dissolved during the subsequent wet-chemical process steps. In the case of planar coating methods such as e.g. spincoating, the entire layer stack can be stripped away and float away, i.e. e.g. slip away, as soon as a solvent in which at least one existing layer is soluble is used for subsequent layer s.
It is conventional practice for at least two solution-processed layer s to be crosslinked with one another in order to improve their connection to one another. However, this reduces the lifetime of the OLED in comparison with vacuum-processed components as a result of the additional materials necessary for the crosslinking or as a result of residues that remain as a result of the crosslinking.
In processing methods with a structuring capability (e.g. slot die coating, inkjet printing or screen printing), passage underneath existing layer s is observed primarily in the edge regions. By way of example, a first layer in polar solvent (e.g. water, dichloromethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP)), thereabove a second layer in apolar solvent (e.g. toluene or xylene) and thereabove a third layer once again in a polar solvent (e.g. alternatives to water such as e.g. dichloromethane, tetrahydrofuran or NMP) are applied. During application layer by layer, the problem occurs that the solvent of the third layer incipiently dissolves the edge regions of the first layer as a result of passage underneath. Analogously thereto, the second layer would be incipiently dissolved in the edge region as a result of passage underneath if a fourth layer once again composed of an apolar solvent is applied. The solvent of the topmost layer passes beyond the region to be coated and incipiently dissolves already existing layer s which have a certain solubility in the new solvent. As a result, undesired mixing can occur primarily in the edge regions (up to a few mm). This adversely affects the functionality of the entire layer stack and the homogeneity of the emission color and also the lifetime of the OLED.
It is conventional practice to construct multilayered components in a solution-processed manner by applying successive layer s from so-called orthogonal solvents (e.g. one polar, the other apolar). In this case, the already existing layer is insoluble in the solvent of the subsequently applied layer. This method is limited to a maximum of four layer s, wherein polar and apolar solvents alternate, or are supplemented by perfluorinated solvents, which represent an intermediate class.
If more layer s are intended to be processed, then the existing layer s are completely stripped away, depending on the coating method. Primarily in the edge regions of the active surfaces it is observed that existing layer s swell and undesired mixing of the layer s takes place, which adversely affects the functionality of the OLED. The difficulty or adverse effect in the edge regions becomes apparent in coating methods with a structuring capability, inter alia. In the case of planar coating methods such as spincoating or dip-coating, the entire layer is often observed to float away.